Explore Introductory Mammalian Physiology with these specialized notes tailored for Year 1 students at University College London. Immerse yourself in the intricacies of cellular physiology and skeletal muscle, delving into the core principles that govern cellular function and the dynamic workings o...
Skeletal Muscle
Skeletal Muscle
Types of muscle
o Skeletal muscle
Striated muscle – due to regular array of contractile elements (actin + myosin = sarcomere)
Activated via action potentials in motor nerves = neurogenic contractions
o Cardiac muscle
Striated muscle
Have intrinsic rhythm that is modulated by action potentials in autonomic nerves =
myogenic contractions
o Smooth muscle
Elongated muscle – no regular array of actin + myosin = no striations
Myogenic contractions
Excitation-contraction
o How a skeletal muscle contraction is initiated
Nerve action potential
ACh secretion by nerve ending
End-plate potential
Muscle action potential
Depolarise T-tubules (transverse) and open Ca2+ channels of SR
T-tubules = plasma membrane that protrudes into skeletal muscle cells
SR = where cells store Ca2+
Sarcoplasmic Ca2+ concentration increases
Contraction
Pump Ca2+ into SR
Relaxation
Innervation
o Skeletal muscles are supplied by myelinated nerve fibres which have their origin in the CNS = motor
nerves
Motor nerve enters muscle fibre = branches – making synaptic contact with many muscle
fibres
o Motor unit
Motor neuron + all the muscle fibres it innervates
Skeletal muscle structure
o Single skeletal muscle contains many muscle fibres
Different components of skeletal muscle –
surrounded by layers of connective tissue
Layers
o Endomysium
A delicate layer of
connective tissue that
surrounds each muscle
fibre
o Perimysium
Connective tissue that surrounds each fascicle
Fascicle = groups of muscle fibre bundles
o Epimysium
Connective tissue that surrounds entire muscle
Muscle membranes
o Myofilaments surrounded by sarcoplasmic reticulum
Arrangement of sarcoplasmic reticulum and T-tubules with myofibrils
,Skeletal Muscle
Filaments
o Thick filaments
Made up of myosin molecules = myosin head + myosin tail
Head = contact point between thick and thin filament
o Thin filament
Actin
Sits on tropomyosin backbone
o Tropomyosin activated by troponin (calcium binding protein)
Calcium binds to troponin = causes 3D shift in protein – exposing
myosin binding sites on actin
o As muscles contract – certain elements can still be identified
Molecules slide against one another
Sarcomere – one functional unit of a muscle fibre = segment between two neighbouring z-lines
o Proteins
Actin filaments = thin filaments
Major component of the I-band + extends
throughout the A-band
Myosin filaments = thick filaments
Extend throughout the A-band
Cross linked at the centre by the M-band
o Bands
Z-lines
Act as an anchoring point of the actin filaments
I-band
Zone of actin filaments that is not superimposed by myosin filaments
A-band
Contains the entire length of single thick filament
Contains both actin and myosin filaments
H-zone
Zone of myosin filaments without actin
M-line
Disc in the middle of the sarcomere – formed from cross-connecting elements of the
cytoskeleton
Contraction cycle
o Myosin heads hydrolyse ATP and become reoriented and energised
o Myosin heads bind to actin = forming crossbridges
o Myosin crossbridges rotate toward centre of sarcomere = power stroke
ADP dissociates
o As myosin heads bind ATP – crossbridges detach from actin
o Contraction cycle continues if ATP is available and Ca 2+ level in sarcoplasm is high
Contractile response
o Contractile response – initiated after the muscle action potential + lasts longer than the action
potential
o
Stimulation frequency
, Skeletal Muscle
o Twitch = individual action potentials resulting in a single contraction
Different frequencies of stimulation affects contractile force
o Summation
Series of few action potentials sent after one another
Does not allow cell to return to initial level = resulting in larger contractile force
o Unfused tetanus
Multiple action potentials sent after one another without allowing cell to return ot initial
level – resulting in larger contractile force
o Tetanus
Increased frequency of action potential – resulting in smooth contrinuous muscle
contraction
o
Contraction
o As sarcomere length decreases below 2 micrometre – the
thin filaments collide in the middle of the sarcomere =
actin myosin interaction is disturbed = contractile force
decreases
o As muscle length is decreased towards optimum length –
the amount of overlap increases = contractile force
increase
o At long sarcomere length – actin filaments do not overlap
with myosin filaments = no contraction
Length-tension relationship
o Muscles contract = generate force – measured as tension or stress + changes in length
Isotonic contractions
Concentric contraction
o Muscle activation that increases the tension on a muscle as it shortens
Eccentric contraction
o Muscle activation that increases tension on a muscle as it lengthens
Isometric contraction
Muscle contraction without any joint movement
No lengthening or contraction of muscles
Type of skeletal muscle
o Fast and slow muscles
Fast muscles – type II
Contract and relax quickly after a single action potential
Tire quickly
Slow muscles – type I
Contract and relax slowly after a single action potential
Tire slowly
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